The present invention relates to a molded article of poly(m-phenylene isophthalamide) (hereinafter referred to as "PMPI") and a process for producing the same.
More in detail, the present invention relates to a molded article excellent in electrical insulation and resistance to chemicals even at a high temperature, which shows a bending strength of higher than 13 kg/mm.sup.2, a density of 1.29 to 1.33 g/cm.sup.3 and a Rockwell hardness of not less than M 120, and a process for producing the same.
Hitherto, as a process for producing molded articles of PMPI, a method of sintering poly(m-phenylene isophthalamide) in an inert atmosphere (ordinarily in nitrogen atmosphere) has been known and adopted as are disclosed in Japanese Patent Publication No. 56-2092 (1981), Japanese Patent Application Laid-Open No. 55-131024 (1980) and Japanese Patent Application Laid-Open No. 57-164 (1982).
The reason why such a sintering method has been adopted is that the practically usable molded articles are very difficultly available by the melt-molding method comprising extruding, injection or compression, because the melting point of PMPI is very high and is close to the decomposition temperature thereof and accordingly, the melting and the decomposition thereof occur nearly simultaneously.
In addition, since PMPI is hygroscopic, when it is subjected to the above-mentioned melt-molding method accompanied by melt-flowing, blisters, cracks or cavities are apt to be formed not only by the gas generated by thermal decomposition of PMPI but also by the vaporization of the moisture contained within PMPI and accordingly, the molded articles produced by the above-mentioned melt-molding method have not been commercialized in market.
On the other hand, it has been generally known that the molded articles produced by sintering method is lower in the mechanical strength as compared to the molded articles produced by melt-molding method.
The mechanical strength of the molded articles produced by sintering method depends greatly on the applied pressure utilized for fabricating preforms of PMPI. More in detail, in the case where the applied pressure is small, the extent of compression of the powder is necessarily small and the porosity of the preform is large and accordingly, the mechanical strength of the molded articles obtained by sintering such a preform is small.
Accordingly, in order to produce the molded articles by sintering method, having high mechanical strength comparative to that of the molded articles by melt-molding method, it is necessary to utilize a remarkably large pressure for fabricating preform of PMPI powder.
Namely, PMPI is subjected to preforming under a high pressure of not less than 700 kg/cm.sup.2, preferably not less than 2,000 kg/cm.sup.2 and more preferably 3,000 to 7,000 kg/cm.sup.2 at a temperature of not more than 250.degree. C., and then the thus obtained preform is sintered at a temperature of 260.degree. to 320.degree. C. in an inert atmosphere at ordinary pressure to be a molded article.
Consequently, in the case of obtaining the molded articles of PMPI by sintering method, the pressure utilized for fabricating preform is a dominant factor and in such a case, it is necessary to install a press of a high output.
In other words, in the case of using an ordinary press, only the molded articles of small in molding surface are available.
In addition to the above-mentioned problems, the sintering method necessitates not only a sintering furnace which can adjust the atmosphere therein by nitrogen but also a long sintering time.
As a result of the present inventors' studies for solving the above-mentioned problems, namely, finding a process for producing molded articles of poly(m-phenylene isophthalamide) excellent in mechanical strength by a lower molding pressure and a shorter molding time than those in the conventional sintering method, the present inventors have found that the molded articles of PMPI of the density of less than 1.29 g/cm.sup.3 have the internal defects such as cavities, blisters or the cracks on the surface thereof with the appearance of frosted graysh white, and the physical properties of such molded articles are low and vary widely.
The above-mentioned defects in the ordinary engineering plastic products are apt to be simply solved as the results of too high or too low of the molding temperature or molding pressure, or the defficiency of drying of molding material.
However, in the case of the heat-resistant high polymer such as aromatic polyamide resin showing a high melting point close to the decomposition temperature thereof and behaviors as thermo-setting synthetic resin, the problems can not be solved by adjusting the temperature and pressure during manufacturing. Namely, it is necessary for solving the problems to collectively examine the bulk density of the powdery resin, the heating and pressing conditions in fabricating preform, the molding conditions thereafter and the cooling conditions of the thus molded articles, etc. in addition to all the factors mentioned above.
As a result of carrying out a number of experiments by varying the bulk density of the PMPI powder, the extent of drying thereof, the pressure and the mold temperature in fabricating preform from the powder and the pressure and the temperature in molding the preform, the present inventors have found that the molded articles produced by the process comprising the steps of
(1) drying PMPI powder having a bulk density of 0.2 to 0.4 g/cm.sup.3, a specific surface area of 2 to 8 m.sup.2 /g and a mean particle size of 37 to 500 .mu.m so that the moisture content thereof becomes to not more than 1% by weight,
(2) fabricating preforms therefrom by compressing the PMPI powder under a pressure of 70 to 500 kg/cm.sup.2 at a temperature of not more than 250.degree. C., and
(3) molding the thus compressed material under a molding pressure of at least 70 kg/cm.sup.2 at a temperature of 290.degree. to 360.degree. C. for 1 to 5 min per 1 mm of the thickness of the thus molded articles, which show a density of 1.29 to 1.33 g/cm.sup.3 and are excellent in mechanical properties with a small fluctuation in physical properties, and the present inventors have attained the present invention based on the findings.